Although the discussion in this specification applies to all optical filters (including any lens, mirror, and any other optical filter), such discussion will sometimes include specific references to the lenses of sunglasses for simplicity. Sunglass lenses are an important embodiment of the inventive optical filter, though by no means is such embodiment the only one contemplated by the inventor.
The common gray, green or brown tinted sunglass is usually a colored glass that absorbs a significant amount of radiation in both the visible and ultra-violet spectra. Tinted glass that has near neutral visible radiation transmission qualities and also totally absorbs ultra-violet or infrared radiation (or both ultra-violet and infrared radiation) is not readily available.
It is well known that clear glass (e.g. BK-7) will transmit the entire visible spectrum, and that a coating may be applied to clear glass in order to reflect certain wavelengths of light. Any wavelength which is completely reflected from such coated glass will not be transmitted. Therefore, these wavelengths will be taken out of the color spectrum when viewing natural objects through the glass.
It is also known that two quarter-wave stacks, where all layers comprising each stack have substantially the same quarter-wave optical thickness, deposited upon a transparent substrate, may be used as a short-wave-pass (SWP) filter. It is conventional to adjust the optical thickness of each stack so that the visible wavelengths (400 nm to 680 nm) will not reflect and the near-infrared (NIR) wavelengths (700 nm to 1100 nm) will totally reflect from the coated substrate. It is also known that a substrate (commonly glass) that absorbs a percentage of the visible wavelengths and all ultra-violet radiation may be employed as a sunglass, and could be coated by a quarter-wave stack of the above-described type. This sunglass will transmit no UV solar radiation and reflect most NIR solar radiation and will allow only a percentage of the total visible spectrum's energy to pass through to the eye.
It is conventional that the double quarter-wave stack may have more than 25 layers whose thickness is optimized so that a very small part of one (or both) of the colors blue and red (on the edges of the visible spectrum) are reflected for aesthetic purposes. Revo.RTM. sunglasses (commercially available through Revo, Inc.), are an example of eyeglasses having such a multi- layer coating. By reflecting only a small amount of the visible radiation, and only reflecting visible radiation in the wavelength regions where the human eye is least sensitive (in the blue and red), a relatively correct color balance is maintained in transmission. This color balance and the observed reflected aesthetic color can be computed numerically and plotted on a C.I.E. 1931 Chromaticity Diagram. Since the glass conventionally used absorbs more blue than red and yellow light, the transmitted color balance tends to be slightly yellow.
If the described multilayer coating were scratched, a visual defect would be observed due to the multilayer interference. However, a thick top layer of fused silica is conventionally used to make most scratches unobservable.
Although such conventional lenses are designed to have desirable optical properties, they are very difficult and expensive to manufacture. Specifically, the design of the prior art lenses require that the coatings must be applied with great accuracy. However, deviations in the manufacturing process commonly cause the reflected wavelengths to vary. If the variation is such that the reflected range is out of the visible region (a common occurrence), the lens will not be properly colored in reflection. If the shift moves the reflected range further into the visible region (another common occurrence), too much visible radiation will be reflected and the transmitted color balance will be lost. This latter result is particulary a problem if red light is reflected so that the wearer might not be able to detect a red traffic light.
Also, since the substrate of this type of prior art lens has a curved surface, a green ring is typically observable (when reflecting red light) near the edge of the lens because the coating's spectral curve shifts toward shorter wavelengths with increasing incidence angle, or coating runoff as the coating is deposited.
More simplistic coating designs are utilized on other commercially available sunglasses. These simple designs typically use basic one to ten layer broad band anti-reflection (AR) coatings. These coatings reflect some part of the visible spectrum for aesthetic purposes. However, they do not simultaneously preserve neutral transmitted color balance and block substantially all near-infrared (NIR) light. If one were to integrate to find the area under the reflectance spectrum of one of these filters (over the visible wavelength band) the total area would be more than one would obtain if one performed similar integration of a reflectance curve characterizing the inventive filter. The quality of the transmitted color balance is accordingly degraded in the conventional coating design. Although the manufacturing complexity of this simple conventional coating design, and the cost to manufacture filters embodying such design, is relatively low, the optical performance of filters embodying such simple coating design is compromised.
The variation of reflected color with viewing angle is another characteristic common to conventional single dielectric and multilayer sunglass coatings (and other multilayer optical filters) that are designed to reflect visible electromagnetic radiation for aesthetic purposes. For example, the above-described Revo.RTM. multilayer coated sunglasses will appear to change in color as a viewer observes them in reflected light from a changing view angle. This chromatic phenomenon occurs generally, except in the special case that the coated sunglasses are designed to reflect only violet light having wavelength less than 470 nm.
It is well known as a matter of optical theory that a multilayer filter's reflectance spectrum (and other characteristics of such a filter) is dependent upon incidence angle, and that this dependence can be affected by changing the thickness of the high and low refractive index layers of the filter. Although this phenomenon is well understood in an abstract sense, until the present invention has not been suggested that the phenomenon could be applied to reduce reflected color variation in quarter wave stack filters, nor in quarter wave stack SWP filters.
It has not been known until the present invention how repeatably and economically manufacturable optical filters (including sunglasses), having a selected aesthetic reflected color and a neutral transmitted color balance, may be designed so that the aesthetic reflected color (and the neutral transmitted color balance) is substantially independent of incidence angle.